This invention relates to the recovery of values, such as gold, from particles of ore in the dry washer. In this process the ore particles are deposited on a riffle table which is covered with a layer of densely woven fabric. The top surface of the riffle table, hereinafter referred to as the core, is densely perforated throughout. The core is also the top, or ceiling, of the air plenum chamber. The core supports the layer of fabric. Compressed air is delivered into the air plenum chamber, from whence it escapes upwardly through the perforations in the core, through the thickness of the fabric layer and through the depth of the bed of particles thereover. The upflow of air tends to fluidize at least the lighter and more buoyant particles in the bed.
One end of the riffle table is elevated to a selected angle. The layer of fabric is held in position by a plurality of about seven transverse, horizontal riffle barriers which engage opposing side walls of the riffle table. The riffle table is walled at the elevated end and at both sides. Means are provided to support the riffle table above ground and at the desired angle but, at the same time, to provide freedom of the table to oscillate with orbital motion coplanarly with the core. An oscillator, rigidly clamped to the air plenum chamber inlet is driven by the flow of air thereinto. It is an eccentric weighted crank on one end of a shaft which is shared by a propeller on the opposite end. The shaft is coaxially aligned with the compressed air duct entering the air plenum chamber.
An air blower powered by an internal combustion engine compresses ambient air for delivery to the air plenum chamber via an interconnecting length of flexible air conduit of dielectric material.
In the prior art, one typical assembly train of this type is described by Thomas Stimpel in U.S. Pat. No. 3,773,174, which issued on Nov. 20, 1973. Stimpel's contribution to the art consists of providing in each perforation in the core of the riffle table a hemispherical bell opening facing the overlying layer of fabric. It is claimed that this provides space for cyclonic whirls which create an electrostatic charge on the fibers of the fabric.
A different approach was taken by Robert H. Bedford in U.S. Pat. No. 2,116,613, which issued on May 10, 1938. In this case the core is a horizontal metal screen connected to the negative lead from an electrostatic generator with the dielectric fabric layer superimposed thereover. The metal screen is electrically insulated from all other component parts of the riffle table/air plenum chamber assembly. The positive lead from the generator is connected to an overhead hopper, which receives the ore particles and delivers them by gravity drop to the riffle table. The particles acquire a positive charge in accordance with their composition, it is claimed, with the metal particles acquiring the higher charge. The Bedford concept treats the separation of the heavier concentrate particles from the lighter particles in the fluidized bed by dividing the bed into two compartments with a dam extending below the surface of the bed but not all the way to the bottom, thus permitting only the heavier fluidized layer of concentrate to pass under the dam, precisely as in the case of separating two immiscible liquids of different densities. The compartment on each side of the dam is provided with its own spillway, wherefrom the heavier values spill over at a lower elevation on one side and the exhausted tailings spill over at a higher elevation on the opposite side. According to Bedford, the electrostatic field is intended to augment the effectiveness of the gravity separation process.
There is no teaching in the cited prior art of heating of the compressed air, of maintaining bone dryness of the fibers in the fabric layer, or of drying the ore particles in the bed, which is the very essence of the present invention.
The improvement of the present invention consists of providing a heat collector shroud to enclose the finned heat radiating surface area and the cylindrical surface of the exhaust muffler of the air-cooled internal combustion engine which powers the air blower. The shroud has a hot air outlet.
In addition, the air inlet to the volute of the blower engages, mouth-to-mouth, the outlet of a blower case. The blower case has a hot air inlet which is connected to the hot air outlet of the heat collector shroud by means of an appropriate length of duct of dielectric material. Thus the heat from the engine that is normally discharged into the atmosphere, is scavenged, according to the invention, to heat up the compressed air to about 50.degree. F. above ambient temperature at no extra cost for fuel.